Internet Engineering Task Force (IETF) R. Ravindranath
Request for Comments: 8124 G. Salgueiro
Category: Standards Track Cisco
ISSN: 2070-1721 March 2017
The Session Description Protocol (SDP)WebSocket Connection URI Attribute
Abstract
The WebSocket protocol enables bidirectional real-time communication
between clients and servers in web-based applications. This document
specifies extensions to Session Description Protocol (SDP) for
application protocols using WebSocket as a transport.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8124.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Ravindranath & Salgueiro Standards Track [Page 1]

RFC 8124 WebSocket SDP Attribute March 2017
For example, [RFC7118] defines a WebSocket subprotocol as a reliable
transport mechanism between Session Initiation Protocol
(SIP)[RFC3261] entities to enable use of SIP in web-oriented
deployments. Additionally, [RFC7977] defines a new WebSocket
subprotocol as a reliable transport mechanism between Message Session
Relay Protocol (MSRP) clients and relays. [RFC7395] defines a
WebSocket subprotocol for the Extensible Messaging and Presence
Protocol (XMPP). Similarly, [BFCP-WEBSOCKET] defines a WebSocket
subprotocol as a reliable transport mechanism between Binary Floor
Control Protocol (BFCP) [BFCP] entities to enable usage of BFCP in
new scenarios.
When a WebSocket subprotocol is used as a transport mechanism between
a server and client, there needs to be a way to indicate the
connection URI from the server to the WebSocket client. For
applications that use Session Description Protocol (SDP) [RFC4566] to
negotiate, the connection URI can be indicated by means of an SDP
attribute. This specification defines new SDP attributes to indicate
the connection URI for the WebSocket client. Applications that use
SDP for negotiation and WebSocket as a transport protocol can use
this specification to advertise the WebSocket client connection URI.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. SDP Considerations3.1. General
Applications that use the SDP Offer/Answer mechanism [RFC3264] for
negotiating media and also use WebSocket or secure WebSocket as a
transport protocol MAY indicate the connection URI for the WebSocket
client via a new SDP "a=" media-level attribute defined in
Section 3.2.
Ravindranath & Salgueiro Standards Track [Page 3]

RFC 8124 WebSocket SDP Attribute March 20173.2. "websocket-uri" SDP Attribute
This section defines a new SDP media-level attribute, "websocket-
uri", which can appear in any of the media sections.
Example:
a=websocket-uri:wss://example.com/chat
Where "wss://example.com/chat" is the ws-URI defined in Section 3 of
[RFC6455].
When the "websocket-uri" attribute is present in the media section of
the SDP, the IP address in "c=" line SHALL be ignored and the full
URI SHALL be used instead to open the WebSocket connection. The
clients MUST ensure that they use the URI to open the WebSocket
connection and ignore the IP address in the "c=" line and the port in
the "m=" line.
3.3. "websocket-uri" Multiplexing Considerations
Multiplexing characteristics of SDP attributes are described in
[SDP-MUX]. Various SDP attribute multiplexing categories are
introduced there.
o The multiplexing category of the "a=websocket-uri" attribute is
CAUTION.
There are no multiplexing rules specified for the "websocket-uri" SDP
media-level attribute. Additionally, the specification of
multiplexing rules for the "websocket-uri" attribute is outside the
scope of this document.
While it is technically possible to bundle WebSocket, there are a
variety of reasons that make it impractical; thus, it is considered
unlikely to be used in practice. Therefore, the "websocket-uri" SDP
media-level attribute defined in Section 3.2 for using WebSocket as a
transport protocol is not likely to be used with SDP bundle and is
consequently categorized as CAUTION for multiplexing.
If future extensions define how to bundle WebSocket, then
multiplexing rules for the "a=websocket-uri" attribute need to be
defined as well, for instance, in an extension of this SDP based
WebSocket negotiation specification.
Ravindranath & Salgueiro Standards Track [Page 4]

RFC 8124 WebSocket SDP Attribute March 20174. SDP Offer/Answer Procedures4.1. General
An endpoint (i.e., both the offerer and the answerer) that wishes to
negotiate WebSocket as transport protocol MUST indicate that it
wishes to use WebSocket or secure WebSocket in the "proto" field of
the "m=" line. Furthermore, the server side, which could be either
the offerer or answerer, MUST add an "a=websocket-uri" attribute in
the media section whose value can be either "ws-URI" or "wss-URI", as
defined in Section 3 of [RFC6455], depending on whether it wishes to
use WebSocket or secure WebSocket. This new attribute MUST follow
the syntax defined in Section 3. The procedures in this section
apply to an "m=" line associated with any media stream that uses
WebSocket or secure WebSocket as transport.
Both offerer or answerer can initiate a WebSocket connection. It is
expected that, based on the topology (for example, if the client is
behind NAT and server is on global IP address), the offerer and
answerer applications decide on who will initiate the WebSocket
connection and appropriately set the "setup" attribute in SDP
following the procedures of [RFC4145].
4.2. Generating the Initial Offer
In order to negotiate WebSocket as a transport, an SDP offerer MUST
indicate that it wishes to use it in the "proto" field of the "m="
line. For example, to negotiate BFCP-over-WebSocket, the "proto"
value in the "m=" line is TCP/WSS/BFCP if WebSocket is over TLS;
else, it is TCP/WS/BFCP, as specified in [BFCP-WEBSOCKET].
The offerer SHOULD assign the SDP "setup" attribute with a value of
"active" (the offerer will be the initiator of the outgoing TCP
connection) or "passive" if the offerer wishes to be a receiver of an
incoming connection. The offerer MUST NOT assign an SDP "setup"
attribute with a "holdconn" value. The offerer MUST follow the
procedures described in [RFC4145] while using the "setup" attribute.
If the "setup" attribute has a value of "passive", it MUST have a URI
in the "a=websocket-uri" attribute.
Ravindranath & Salgueiro Standards Track [Page 5]

RFC 8124 WebSocket SDP Attribute March 2017
The following is an example of an "m=" line for a BFCP connection:
Offer (browser):
m=application 9 TCP/WSS/BFCP *
a=setup:active
a=connection:new
a=floorctrl:c-only
m=audio 55000 RTP/AVP 0
m=video 55002 RTP/AVP 31
In the above example, the client is intending to set up the TLS/TCP
connection; hence, the port is set to a value of 9, which is the
discard port.
4.3. Generating the Answer
If the answerer accepts the offered WebSocket transport connection,
in the associated SDP answer, the answerer MUST assign an SDP "setup"
attribute with a value of either "active" or "passive", according to
the procedures in [RFC4145]. The answerer MUST NOT assign an SDP
"setup" attribute with a value of "holdconn".
If the answerer assigns an SDP "setup" attribute with a value of
"active", the answerer MUST initiate the WebSocket connection
handshake by acting as client on the negotiated media stream, towards
the URI specified in the "a=websocket-uri" SDP attribute using the
procedures described in Section 4 of [RFC6455].
If the answerer assigns an SDP "setup" attribute with a value of
"passive", then it MUST have a value of "ws-URI" or "wss-URI", as
defined in Section 3 of [RFC6455] in an "a=websocket-uri" SDP
attribute, depending on whether the application uses WebSocket or
secure WebSocket. This attribute MUST follow the syntax defined in
Section 3.
Ravindranath & Salgueiro Standards Track [Page 6]

RFC 8124 WebSocket SDP Attribute March 2017
The following example shows a case where the server responds with a
BFCP media stream over a WebSocket connection running TLS. It shows
an answer "m=" line for the BFCP connection. In this example, since
WebSocket is running over TLS, the server answers back with an
"a=websocket-uri" attribute in the media section of SDP having a
"wss-URI" connection URI:
Answer (server):
m=application 50000 TCP/WSS/BFCP *
a=setup:passive
a=connection:new
a=websocket-uri:wss://bfcp-ws.example.com?token=3170449312
a=floorctrl:s-only
a=confid:4321
a=userid:1234
a=floorid:1 m-stream:10
a=floorid:2 m-stream:11
m=audio 50002 RTP/AVP 0
a=label:10
m=video 50004 RTP/AVP 31
a=label:11
4.4. Offerer Processing of the Answer
When the offerer receives an SDP answer, if the offerer ends up
initiating the TCP connection, then it MUST follow the procedures in
Section 5.
4.5. Modifying the Session
Once an offer/answer exchange has been completed, either endpoint MAY
send a new offer in order to modify the session. The endpoints can
reuse the existing WebSocket connection by adding an
"a=connection:existing" attribute in the media section of the SDP
following the rules mentioned in [RFC4145], if the "websocket-uri"
SDP value and the transport parameters indicated by each endpoint are
unchanged. Otherwise, following the rules for the initial offer/
answer exchange, the endpoints can negotiate and create a new
WebSocket connection on top of TLS/TCP or TCP.
Ravindranath & Salgueiro Standards Track [Page 7]

RFC 8124 WebSocket SDP Attribute March 20174.6. Offerless INVITE Scenarios
In some scenarios, an endpoint (e.g., a browser) originating the call
(a User Agent Client or UAC) can send an offerless INVITE to the
server. The server will generate an offer in response to the INVITE.
In such cases, the server MUST send an offer with the "setup"
attribute with a value of "passive" so as to accept incoming
connection and MUST include an "a=websocket-uri" attribute in the
media section whose value MUST be either "ws-URI" or "wss-URI",
depending on whether the server wishes to use WebSocket or secure
WebSocket. The SDP offer sent by the server will look like the
example in Section 4.3.
5. Procedures at WebSocket Client
The WebSocket client MUST always initiate the outgoing TCP
connection; hence, the SDP "setup" attribute MUST always be "active"
for the WebSocket client in its SDP offer/answer. In the example
below, the WebSocket client is the offerer; hence, it assigns a
"setup" attribute with a value of "active".
The WebSocket server is a server on the Internet; hence, it MUST
always assign an SDP "setup" attribute with a value of "passive".
This also avoids the need to use Interactive Connectivity
Establishment (ICE) between WebSocket client and WebSocket server, as
the connection model here would be a typical client-to-server web
connection.
Once the offer/answer is complete, the client MUST initiate the
WebSocket connection handshake by sending a GET message on the
negotiated media stream, towards the URI specified in an
"a=websocket-uri" attribute, as per the procedures described in
[RFC6455]. When no port is passed in the "a=websocket-uri"
attribute, the default port (80 or 443) is used depending on whether
the value was "ws-URI" or "wss-URI".
Ravindranath & Salgueiro Standards Track [Page 8]

RFC 8124 WebSocket SDP Attribute March 20176. Security Considerations
An attacker may attempt to add, modify, or remove an
"a=websocket-uri" attribute from a session description. This could
result in an application behaving undesirably. Consequently, it is
RECOMMENDED that integrity protection be applied to the SDP session
descriptions. For session descriptions carried in SIP [RFC3261],
S/MIME is available to provide such end-to-end integrity protection.
As described in Section 10 of [RFC6455], application signalling
traffic being transported over WebSocket MUST support secure
WebSocket and SHOULD employ it when communicating with their peers.
The WebSocket clients have to initiate the TCP connection to the
WebSocket server identified by the Fully Qualified Domain Name (FQDN)
in an "a=websocket-uri" attribute. Further, as with any other web
connection, the clients will verify the server's certificate. The
WebSocket client MUST follow the procedures in [RFC7525] (including
host name verification as per Section 6.1 in [RFC7525]) while setting
up a TLS connection with a WebSocket server.
7. IANA Considerations7.1. Registration of the "websocket-uri" SDP Media Attribute
This document defines a new SDP media-level attribute "websocket-uri"
in Section 3.2; IANA has registered the following SDP att-field under
the "Session Description Protocol (SDP) Parameters" registry as
follows:
+---------------------+---------------------------------------------+
| Attribute name: | websocket-uri |
| Long-form attribute | WebSocket Connection URI |
| name: | |
| Type of attribute: | media |
| Mux category: | CAUTION |
| Charset Dependent: | No |
| Purpose: | The "websocket-uri" attribute is intended |
| | to be used as a connection URI for opening |
| | the WebSocket connection. |
| Appropriate values: | A ws-URI or wss-URI, as defined in Section |
| | 3 of [RFC6455] |
| Contact name: | Gonzalo Salgueiro |
| Contact email: | gsalguei@cisco.com |
| Reference: | RFC 8124 |
+---------------------+---------------------------------------------+
Ravindranath & Salgueiro Standards Track [Page 9]